But many species have run out of suitable habitat or fallen prey to pests and disease, while others are suffering from extreme weather events such as El Niños—global climate disruptions that have increased in intensity and severity since the early 1900s.

One El Niño in 1997-1998 caused 16 percent of global corals to go extinct, which in turn threatened many fish species.

"Fish depend on the structure coral reefs provide," said biologist Boris Worm of Dalhousie University in Halifax, Canada.

For species such as coral, the extreme swings in temperature that can be caused by global warming are more of a concern than the rising average temperatures, Worm said.

Harlequin frogs native to the cloud forests of Costa Rica have been hit especially hard.

In January J. Alan Pounds, a resident scientist at Costa Rica's Monteverde Cloud Forest Preserve, reported that about two-thirds of the 110 known harlequin frog species had been killed off by a disease-causing fungus. (Related: "'Frog Hotel' to Shelter Panama Species From Lethal Fungus" [November 2, 2006].)

The fungus thrives in warmer temperatures, which also make frogs more susceptible to infection.

On Thin Ice

In the Antarctic, three decades of declining sea ice have led to a reduction of ice algae. This, in turn, has reduced the number of krill, an essential food for many fish, marine mammals, and seabirds, including penguins.

"We've predicted that the Adélie penguin will soon be locally extinct," Fraser, of the Polar Oceans Research Group, said.

The species has already nearly disappeared from its northernmost sites in the Antarctic. The population on Anvers Island, for example, has declined more than 70 percent, from 16,000 breeding pairs 30 years ago to 3,500 today (map of Antarctica).

And this year the Adélie population on Litchfield Island disappeared.

"It is the first time in 700 years that the island does not have penguins on it," Fraser said.

"The arctic ice is reducing in area and thickness—some places are just too thin to support a polar bear," the University of Texas's Parmesan said.

Such animal woes may hint at hard times ahead for humans, Fraser added.

"The planet has warmed and cooled in the past, but never have we seen the type of warming that is occurring now, accompanied by the presence of 6.5 billion people who depend on these ecosystems," he said.

"Whether we want to admit it or not, we are completely and totally dependent on them."